Non-dispersive ir measurement of gases using an optical filter

ABSTRACT

A method and apparatus are described for determining the safety of a gas mixture containing flammable components such as methane together with ethane or other hydrocarbon, together with a diluent gas. The method comprises the filtered infrared spectroscopy of the gas mixture in a gas cell ( 10 ) using a filter ( 16 ). The peak transmission wavelength (λ max ) and bandwidth of the filter ( 16 ) are so chosen to provide an output, when an infrared light source ( 42 ) having a flat wavelength distribution is used, indicative of the %LEL of the gas mixture, within a predetermined tolerance. The filter may be a gas correlation filter containing a mixture of methane and ethane, together with a diluent gas.

FIELD OF THE INVENTION

[0001] The present invention relates to various aspects of determiningthe safety of a gas mixture which contains flammable components byinfrared spectroscopy, especially but not exclusively where the gasmixture contains methane and ethane in unknown proportions.

BACKGROUND OF THE INVENTION

[0002] Flammable gas concentration measurements are made in a number ofsafety-critical situations. One such flammable gas is natural gas, whichtypically comprises mainly methane, plus higher hydrocarbons, inertgases and trace components. Natural gas detectors are needed for anumber of applications including response to public reported gas escapesand for continuous monitoring of plant/equipment using permanentlyinstalled detectors. They are required to measure the gas concentrationas a percentage of the lower explosion limit (LEL) of the gas mixture,this being an important safety parameter.

[0003] The concentration of a flammable gas component, such as methane,in a gas mixture can, theoretically, be measured by infraredspectroscopy, using a filter having a peak transmission wavelength equalto one of the wavelengths of absorption by methane, e.g. at 3.32 μm. Thefilter and the light source together define a selected range ofwavelengths over which the spectral measurement of the gas mixture ismade, giving a degree of selectivity for individual gas species. Themeasured concentration can be converted into %LEL to give an indicationof the safety of the gas mixture. However, gas mixtures from naturalsources will usually contain unknown proportions of other flammablecomponents, such as hydrocarbons including ethane, propane and butane,while in some cases such components are deliberately added. The presenceof these additional flammable components disturbs the accuracy of the%LEL measurement, by absorbing infrared radiation to a degree which isout of proportion with their influence on LEL, relative to methane.

SUMMARY OF THE INVENTION

[0004] It is an object of the present invention to provide a filter foruse in the infrared spectroscopy of a gas mixture, which enables a moreaccurate indication of %LEL to be achieved.

[0005] We have discovered that not only the peak transmission wavelengthof the filter, but also its band width are important and that improvedaccuracy can be achieved by suitable selection of these factors.

[0006] Thus, according to a first aspect of the invention, there isprovided a method for determining the safety of a gas mixture containingfirst and second flammable components, together with a diluent gas,comprising the filtered infrared spectroscopy of the gas mixture using afilter, the peak transmission wavelength (λ_(max)) and bandwidth ofwhich are so chosen to provide an output indicative of the %LEL of thegas mixture, within a predetermined tolerance.

[0007] The invention is particularly advantageous where the twoflammable components exhibit some absorption of infrared in the sameregion of the spectrum. Typical examples are components having a somechemical similarity, such as members of the same chemical series. Thus,the first flammable component will typically be methane and the secondflammable component will be ethane, propane or a mixture thereof. Whenthe first component is methane, the invention is less successful wherethe second component is hydrogen.

[0008] The diluent components present in the gas mixture will inpractise usually be air, i.e. nitrogen, oxygen, carbon dioxide, watervapour and inert gases, but the invention is applicable to gas mixtureswhere the diluent components do not have a significant infraredabsorption close to the peak transmission wavelength of the filter. Gascomponents which do have a significant infrared absorption close to thepeak transmission wavelength of the filter will disturb the measurementand are therefore preferably avoided.

[0009] The filter is preferably so chosen that the output is indicativeof the %LEL for the mixture, within a tolerance of ±3%.

[0010] We have found that this accuracy can be achieved by theapplication of certain criteria to the selection of the filter.

[0011] Firstly, we have found that it is preferred that the peaktransmission wavelength and the band width of the filter are so chosenthat, when viewed through said filter, the intensity of transmission(I₁) through a gas mixture containing 50% LEL of said first component isequal to the intensity of transmission (I₂) through a gas mixturecontaining 50% LEL of said second component, within a tolerance of 30%,preferably within 20%, ideally within 10%. For mixtures of methane andethane in air,

[0012] I_(methane)≈I_(ethane)

[0013] occurs, depending upon the band width, at wavelengths of about3.27 μm and about 3.32 μm in the mid infrared region, and at wavelengthsof about 1.67 μm in the near infrared region.

[0014] A second preferred criterion is that, at the peak transmissionwavelength of the filter, the rate of change of intensity withincreasing wavelength (δI₁/δλ) for the gas mixture containing 50% LEL ofsaid first component is equal to the rate of change of intensity withincreasing wavelength (δI₂/δλ) for the gas mixture containing 50% LEL ofsaid second component, within a tolerance of 100 I/μm, preferably within10 I/μm. For mixtures of methane and ethane in air we have found that

[0015] δI_(methane)/δλ≈I_(ethane)/δλ

[0016] and that

[0017] I_(methane≈I) _(ethane)

[0018] occurs at a wavelength of about 3.32 μm and a band width,expressed in terms of full width at half maximum, of less than 0.7%λ_(max).

[0019] Thus, in the mid infrared region we prefer that the filter has apeak transmission wavelength λ_(max) of (i) from 3.263 to 3.271 μm, mostpreferably from 3.265 to 3.269 μm, with a bandwidth of between 0.8% and1%, or (ii) between 3.31 and 3.32 μm, with a bandwidth of less than0.7%. In the near infrared region we prefer that the filter has a peaktransmission wavelength λ_(max) of from 1.67 to 1.68 μm, most preferablyfrom 1.673 to 1.675 μm, with a bandwidth of between 0.5% and 6%.

[0020] A filter having λ_(max)=3.27 μm and a band width of 0.9% λ_(max)is commercially available from NDC Infrared Engineering of GallifordRoad, Malden, Essex, UK. The same manufacturers can also provide afilter having λ_(max)=1.67 μm and a band width of 0.9% λ_(max.) Morepreferred filters can be manufactured with suitable adjustments to knownprocessing techniques, or by selection from a variety of filters, toprovide a product with the desired characteristics.

[0021] It is indeed surprising that, in the mid infrared region,reducing the band width of the filter improves the accuracy of the % LELmeasurement, since reducing the band width significantly reduces signalstrength. It is also surprising that moving λ_(max) to a position whereI_(methanae) and I_(ethanae) are substantially equal, at a band width ofless than 0.7% λ_(max), improves the accuracy of the %LEL measurement.

[0022] The invention provides the advantage that the preferred filtercharacteristics are independent of relative proportions of gases in thegas mixture to be examined.

[0023] According to a second aspect, the invention provides an apparatusfor determining the safety of a gas mixture containing first and secondflammable components, together with a diluent gas, the apparatuscomprising a region for receiving gas to be examined, an infrared lightsource positioned to direct infrared light through said region, a sensorfor measuring the intensity of light passed through said region and afilter, positioned in the light path between the source and the sensor,characterised in that the peak transmission wavelength (λ_(max)) andbandwidth of the filter are so chosen to provide the sensor with anoutput indicative of the %LEL of the gas mixture, within thepredetermined tolerance.

[0024] The region for receiving gas to be examined may be provided by agas cell for containing a sample of such gas, or be provided by an openoptical path through which gas to be examined can flow.

[0025] The invention also provides a filter for use in the infraredspectroscopy of a gas mixture containing methane as a first componentand a second component selected from ethane, propane and mixturesthereof, together with a diluent gas, characterised in that the peaktransmission wavelength (λ_(max)) and bandwidth of the filter is such asto provide an output, when an infrared light source having a flatwavelength distribution is used, indicative of the %LEL of the gasmixture, within a predetermined tolerance.

[0026] The nature of the infrared light source is a secondaryconsideration. In theory, if the light source has a “white” output, thatis a flat wavelength distribution in that part of the spectrum beingexamined, then it has no effect upon the preferred characteristics ofthe filter. However, in practice, the infrared light source may not havea flat distribution, particularly if an LED is used as the light source.In this event, it is preferred to select the filter characteristics withthe characteristics of the infrared light source in mind. Similarconsiderations also apply to the sensor.

[0027] Thus, also provided by the invention is the combination of aninfrared light source and a filter for use in the infrared spectroscopyof a gas mixture containing methane as a first component and a secondcomponent selected from ethane, propane and mixtures thereof, togetherwith a diluent gas, characterised in that the peak transmissionwavelength (λ_(max)) and bandwidth of the filter is such as to providean output, when the light source is used, indicative of the %LEL of thegas mixture, within the predetermined tolerance.

[0028] While interference filters are suitable for use in the invention,a gas correlation filter may alternatively be used.

[0029] Thus, in an alternative embodiment, the filter comprises a gascorrelation filter containing a known mixture of the first and secondflammable components, together with a diluent gas.

[0030] The invention still further provides a gas correlation filter foruse in the infrared spectroscopy of a natural gas, the filter containinga mixture of methane and a second flammable component selected fromethane, propane and mixtures thereof, together with a diluent gas.

[0031] The invention will now be illustrated, purely by way of example,by reference to the accompanying drawings, in which:

[0032]FIG. 1 is a schematic representation of an apparatus fordetermining the safety of a gas mixture;

[0033]FIG. 2 is a simulation graph showing the output signal at 50%LELfor methane and ethane using a mid infrared filter with a band width of0.9%λ_(max);

[0034]FIG. 3 is a simulation graph showing the output signal at 50%LELfor methane and ethane using a mid infrared filter with a band width of0.6%λ_(max).; and

[0035]FIGS. 4a and 4 b show actual experimental results obtained usingan interference filter with peak transmission at 3.266 μμm and aninterference filter with peak transmission at 3.324 μm, respectively, tomeasure the concentration of various gas mixtures on the %LEL scale.

[0036] Referring to FIG. 1, there is shown an apparatus for determiningthe safety of a gas mixture containing first and second flammablecomponents, together with a diluent gas. The apparatus comprises aregion for receiving gas to be examined provided by a gas cell 10 forcontaining such a sample of gas. An infrared light source 12 ispositioned to direct infrared light through the gas sample in the cell10. A suitable infrared light source is Chemled LED 33, ex TelecomDevices Corporation, available through Access Pacific Ltd,Wellingborough, Northants, UK. A sensor 14 is provided for measuring theintensity of light passed through the gas sample in the cell. A suitablesensor is P791-11 PbSe photodetector ex Hamamatsu Photonics UK Ltd,Enfield, UK. A filter 16 is positioned between the IR light source 12and the cell 10, but may in an alternative configuration be positionedbetween the cell 10 and the sensor 14. Lenses 18 and 20 are provided toensure that the light from the source 12 is focussed onto the sensor 14.Selection of a near infrared light source and detector, together withthe near infrared filters described above, would also result in anacceptable apparatus.

[0037] Referring to FIG. 2, it can be seen that between the wavelengthsof 3.1 μm and 3.55 μm, the absorption spectra of methane (line M) andethane (line E) are very different. However, they are found to cross atpoint A, at a wavelength of about 3.267 μm. This Figure indicatespreferred characteristics for the filter, namely a peak transmissionwavelength of about 3.267 μm. However, at this wavelength, the slope ofeach line is not similar, i.e. the rate of change of intensity withincreasing wavelength for the two gases is different. This does nottherefore indicate the most preferred characteristics for the filter,for which reference should be made to FIG. 3. However, close examinationof the slopes of the lines at point A, will indicate that a band widthof 0.9%λ_(max) or less will lead to an error in the measurement of %LELfor a 90/10 methane/ethane mixture of no more than 3%.

[0038] Referring to FIG. 3, where the band width of the filter isreduced to 0.6%, it can be seen that there is now a second region B inwhich the lines M and E are close to each other. This is at a wavelengthof about 3.32 μm. Furthermore, at this wavelength, the slope of eachline is similar, i.e. the rate of change of intensity with increasingwavelength for the two gases is substantially equal. This Figureindicates the most preferred characteristics for the filter, namely apeak transmission wavelength of about 3.315 μm and a band width of about0.6%λ_(max).

[0039] It can also be seen from FIG. 3, that the overall signal strengthis reduced, compared to FIG. 2.

EXPERIMENTAL EXAMPLE

[0040] Experiments will now be described that confirm the practicalapplication of the previous simulation analysis. By way of example,experiments were conducted using the mid infrared filters describedabove, but the principle is equally applicable to near infraredoperation.

[0041] A laboratory FTIR spectrometer (Biorad FTS-60A) was used todemonstrate the benefit of choosing filters referred to above. Theconcentration of a series of test gas mixtures was established using twointerference filters, the mixtures being indicative of natural gascompositions found in the UK. The test gas mixtures had the compositionsgiven in Table 1. TABLE 1 Compositions in mol % of three artificial gasmixtures typical of natural gas. LELs have been calculated according tothe method given by Coward and Jones using LELs of individual componentsfrom BS EN 50054: 1991. (Reference: H F Coward and G W Jones. Limits offlammability of gases and vapours. National Bureau of Mines, Bulletin503 [1952]) Gas component Composition 1 Composition 2 Composition 3Nitrogen 1.72 0.731 2.21 Carbon dioxide 0.32 2.11 0.8 Methane 93.5586.48 92.86 ethane 3.27 7.47 3.02 propane 0.763 2.5 0.635 i-butane 0.1220.182 0.131 n-butane 0.153 0.392 0.156 n-pentane 0.103 0.13 0.18 C6+ 00.0003 0.0002 LEL/% vol 4.89 % vol 4.63 % vol 4.94 % vol

[0042] The natural gases at 100% were blended with hydrocarbon free airin varying proportions. The concentration was determined using a methaneanalyser (ADC dual Luft cell), which was separately adjusted for thecross-sensitivity to the other components of each gas mixture to give anaccurate reading for each.

[0043] Gas spectra were measured using a 10 cm pathlength gas cell in alaboratory FTIR spectrometer. Spectra were measured in the mid infrared(centred around 3.3 μm). The spectrometer (Bio-Rad FTS-60A) was set upaccording to the manufacturer's instructions, for high resolution midinfrared spectroscopy. A high temperature ceramic light source was used,with a wide band KBr beamsplitter and liquid nitrogen cooled MCTdetector, all of these being supplied with the spectrometer. KBr windowswere also used in the gas cell. The highest available resolution (0.25cm⁻¹) was chosen.

[0044] Spectra from the sample gases were corrected for cell absorptionsand reflections by subtraction of a reference spectrum, taken with thecell filled with hydrocarbon free air. For each spectrum, a baselinezero was established by interpolation of a straight line between theaverage absorption in the following two regions: (i) 3.0-3.1 μm, and(ii) 3.9-4 μm. These regions were chosen for their insignificant levelsof absorption for natural gas. Baseline zero reference measurements,made using carefully selected filters in regions unaffected by gasabsorption, are well-known in non-dispersive infrared gas detectors.

[0045] The measurement performance of two different interference flterswas compared. The first was chosen according to the previous text, tohave a transmission peak close to 3.267 μm (actually 3.266 μ) and a fwhmbandwidth of under 0.9% (actually 0.81%). The second was chosen so as tomaximise the signal from methane, with a transmission peak at 3.324 μmthat corresponded with the maximum available methane signal, and a fwhmbandwidth of 0.83%.

[0046] (All figures provided by the manufacturer, NDC InfraredEngineering, Maldon, Essex, UK.)

[0047] The transmission spectra of each of the two interference filterswas measured separately using the FTIR spectrometer with the sameconfiguration settings as before. The effect of using each filter tomake a non-dispersive measurement of gas concentration was thenevaluated as follows.

[0048] Working in the transmission domain, the transmission spectrum ofone of the filters was multiplied by the transmission spectrum of one ofthe gas mixtures.

[0049] This gave a signal equivalent to the transmission spectrum whenthe filter and gas cell were placed in series in the optical path of thespectrometer. The total amount of light that would pass through thecell/filter in this circumstance was calculated by integrating the lighttransmission in a broad window from 3.0 to 3.7 μm. This gave a signalequivalent to that measured by a single non-dispersive gas detectorwhose spectral selection of the gas absorption was determined by theinterference filter alone.

[0050] This analysis was repeated for every combination of each of thetwo filters and three gas mixtures,plus a methane control, over a rangeof concentrations covering the %LEL scale.

[0051] The synthesised signals, in arbitrary units, were larger whenusing the second interference filter than when using a firstinterference filter. A single calibration factor was therefore appliedto all the data obtained using each interference filter. This factor waschosen so as to give accurate results for the methane control gas athigh concentrations.

[0052] Experimental Results

[0053] At a range of gas concentrations, the signals obtained for thedifferent gas mixtures as set out in Table 1 when using each of the twointerference filters are shown in FIGS. 4a and 4 b.

[0054] A degree of nonlinearity can be observed in the results in FIGS.4a and 4 b, as a consequence of saturation effects associated withBeer's Law at high absorption levels. The degree of nonlinearity isgreater in FIG. 4b, which is consistent with the second interferencefilter selecting a range of absorption lines with greater levels ofabsorption. Such nonlinearities can be reduced by using a calibrationlook-up table or by using a shorter optical pathlength through the gascell.

[0055] It is clear from FIGS. 4a and 4 b that use of the firstinterference filter at 3.266 μm (FIG. 4a) has resulted in a small spreadof results for different gas mixtures at the same concentration. Incontrast, the spread of results found when using the second interferencefilter (FIG. 4b) is much greater. When using gas detectors to quantifynatural gas leaks, the composition of natural gas, to a degreerepresented by the above data, is not known. If calibrated using amethane only reference, a gas detector based on the second filter couldoverestimate the level of natural gas in real gas leaks by up to 100%because of inappropriate cross-sensitivity to the non-methane componentsof the natural gas.

[0056] The range of proportional errors associated with each filter wascalculated for each gas concentration used. The average error range forthe first interference filter was 10%, while that for the second filterwas 36%. Even if methane is excluded from the analysis, the mean errorrange with the first filter is 5% compared to 19% with the second. It isclear that an appropriately chosen filter can significantly reduce thelevel of composition-related error for gas detectors based onnon-dispersive infrared measurements.

1. A method for determining the safety of a gas mixture containing firstand second flammable components, together with a diluent gas, comprisingthe filtered infrared spectroscopy of the gas mixture using a filter,the peak transmission wavelength (λ_(max)) and bandwidth of which are sochosen to provide an output indicative of the %LEL of the gas mixture,within a predetermined tolerance.
 2. A method according to claim 1,wherein said first flammable component is methane.
 3. A method accordingto claim 2, wherein said second flammable component is selected fromethane, propane and mixtures thereof.
 4. A method according to anypreceding claim, wherein said output is indicative of the %LEL for themixture, within a tolerance of ±3%.
 5. An apparatus for determining thesafety of a gas mixture containing first and second flammablecomponents, together with a diluent gas, the apparatus comprising aregion for receiving gas to be examined, an infrared light sourcepositioned to direct infrared light through said region, a sensor formeasuring the intensity of light passed through said region and afilter, positioned in the light path between said source and saidsensor, characterised in that the peak transmission wavelength (λ_(max))and bandwidth of said filter are so chosen to provide said sensor withan output indicative of the %LEL of the gas mixture, within apredetermined tolerance.
 6. An apparatus according to claim 5, in whichthe gas region is provided by a gas cell for containing a sample of gasto be examined.
 7. An apparatus according to claim 5, in which theregion is provided by an open optical path through which gas to beexamined can flow.
 8. An apparatus according to claim 5, 6 or 7, whereinthe peak transmission wavelength and the band width of the filter are sochosen that, when viewed through said filter, the intensity oftransmission through a gas mixture containing 50%LEL of said firstcomponent is equal to the intensity of transmission through a gasmixture containing 50%LEL of said second component, within a toleranceof 30%.
 9. An apparatus according to any of claims 5 to 8, wherein saidfilter comprises a gas correlation filter containing a known mixture ofsaid first and second flammable components, together with a diluent gas,to provide said sensor with an output indicative of the %LEL of the gasmixture, within a predetermined tolerance.
 10. A method according toclaim 9, wherein, at the peak transmission wavelength of the filter, therate of change of intensity with increasing wavelength for the gasmixture containing 50% LEL of said first component is equal to the rateof change of intensity with increasing wavelength for the gas mixturecontaining 50% LEL of said second component, within a tolerance of 100times the signal level per μm.
 11. A filter for use in the infraredspectroscopy of a gas mixture containing methane as a first componentand a second component selected from ethane, propane and mixturesthereof, together with a diluent gas, characterised in that the peaktransmission wavelength (λ_(max)) and bandwidth of the filter is such asto provide an output, when an infrared light source having a flatwavelength distribution is used, indicative of the %LEL of the gasmixture, within a predetermined tolerance.
 12. A filter according toclaim 11, wherein said filter has a peak transmission wavelength λ_(max)of from 3.265 to 3.269 μm.
 13. A filter according to claim 12, whereinsaid filter has a band width, expressed in terms of full width at halfmaximum, of less than 0.9%λ_(max).
 14. A filter according to claim 11,wherein said filter has a peak transmission wavelength λ_(max) of from3.31 to 3.32 μm.
 15. A filter according to claim 14, wherein said filterhas a band width, expressed in terms of full width at half maximum, ofless than 0.7%λ_(max).
 16. A filter according to claim 11, wherein saidfilter has a peak transmission wavelength λ_(max) of from 1.673 to 1.675μm.
 17. A filter according to claim 16, wherein said filter has a bandwidth, expressed in terms of full width at half maximum, of between 0.5%and 6%λ_(max).
 18. The combination of an infrared light source and afilter for use in the infrared spectroscopy of a gas mixture containingmethane as a first component and a second component selected fromethane, propane and mixtures thereof, together with a diluent gas,characterised in that the peak transmission wavelength (λ_(max)) andbandwidth of the filter is such as to provide an output, when said lightsource is used, indicative of the %LEL of the gas mixture, within apredetermined tolerance.
 19. A gas correlation filter for use in theinfrared spectroscopy of a natural gas, said filter containing a mixtureof methane and a second flammable component selected from ethane,propane and mixtures thereof, together with a diluent gas, in quantitiesthat give equal signals from a mixture of methane/air at 50%LEL and froma mixture of the second flammable component and air at 50%LEL, within atolerance of 10%.
 20. A method for determining the safety of a gasmixture substantially as herein described.
 21. An apparatus fordetermining the safety of a gas mixture substantially as hereindescribed.
 22. A filter for use in the infrared spectroscopy of a gasmixture containing methane as a first component and a second componentselected from ethane, propane and mixtures thereof, substantially asherein described.
 23. A gas correlation filter for use in the infraredspectroscopy of a gas mixture containing methane as a first componentand a second component selected from ethane, propane and mixturesthereof, substantially as herein described.
 24. The combination of aninfrared light source and a filter for use in the infrared spectroscopyof a gas mixture containing methane as a first component and a secondcomponent selected from ethane, propane and mixtures thereof,substantially as herein described.